Such a tensioning device is known from DE 19631607 A1. The screw-in chain adjuster described in this citation comprises, according to one embodiment, a screw-in housing with an inserted tube in which a tubular piston is guided to be longitudinally movable. The tube is provided with a bottom in which a check valve is arranged. The tube and the piston define a chamber space for hydraulic liquid, in particular engine oil. A damping means is arranged at the front end of the piston. The damping means comprises a roller which is arranged coaxially with respect to the piston. In the region of the roller, the piston's interior is configured as a seat for the roller. The receiving wall and the surface area of the roller here define a leakage gap. The inner wall of the piston is provided with an annular groove axially adjacent to the piston bottom corresponding to a vent opening provided in the piston bottom. The leakage gap essentially influences damping. This means that another leakage gap is formed between the piston and the tube, which is, however, selected to be narrow enough to represent a negligible damping influence, and damping is essentially influenced by the leakage gap between the roller and the piston.
Such damping means inside the piston are rather an exception, as in most tensioning devices, in particular chain adjusters, damping is performed by means of a leakage gap between the piston and the housing or the housing tube, and a vent means in communication with a vent opening in the piston is provided inside the piston. One component of such vent means often is a packing element which in most cases has the shape of a mushroom. This packing element is pressed against the end side of the hollow space in the tensioning piston by means of a pressure spring arranged inside the pressure chamber. Depending on the design, air can only escape to the vent opening via a gap in the packing element. The packing element mainly also serves to reduce the volume of the pressure chamber, so that for filling it completely, e.g. when an internal combustion engine is started, only a small amount of oil is required for completely filling the pressure chamber.
Tensioning devices having such damping means are in most cases connected to the engine oil circuit and naturally exhibit certain leakage during operation, so that the lost amount of oil must be constantly refilled via the check valve. The amount of oil to be circulated here must also be taken into consideration when designing the oil pump and determining its flow rate.